1. Field of the Invention
This invention relates to a method of ion nitriding.
2. Description of the Prior Art
As a means of improving fatigue limit, pitting-resistance, etc. of steel structural parts for machine (workpiece), such as a transmission gear for automobile, a carburizing and quenching treatment has been known widely. However, the carburizing and quenching requires austenitization of a workpiece. This austenitization results in causing the so-called thermal strain on a workpiece and in the case of transmission gear, it can cause vibration and noises.
On the other hand, as a means of a surface treating means which involves less thermal strain, a method of ion nitriding has been known. In the conventional ion nitriding method, however, a nitrogen compound layer (Fe.sub.4 N) is formed at the outermost surface of a workpiece and a nitrogen diffusion layer is formed thereunder. As this nitrogen compound layer is hard and brittle, in the case of the transmission gear which is used under severe conditions, for example, the nitrogen compound layer (about 15.mu.) at the surface exfoliates under high stress and abnormal wear takes place. Furthermore, the nitrogen compound layer cracks and such cracks spread to the diffusion layer, with the result that pitting is caused. As compared with a workpiece subjected to a carburizing treatment, a workpiece subjected to a nitriding treatment is thin in its hardened layer at the surface and therefore plastic deformation takes place by external force at or about a boundary between the hardened layer and a base metal. Thus, internal cracks occur and spalling is caused.
Japanese Patent Application Publication Gazette No. 61-31184 refers to a soft-nitriding treatment, more particularly, after a workpiece was hot-processed, metal structure is adjusted by controlled cooling and then a nitriding treatment is carried out. According to this method, it is possible to enlarge the hardened depth from the surface of a workpiece and to improve pitting-resistance and so on.
In the above soft nitriding treatment, however, a pre-treatment of a workpiece becomes complicated. It may be possible to deepen the surface hardened layer by making the nitriding treatment duration longer but this is unfavorable from productivity point of view.